Transistor oscillator starting circuit



Sept. 30, 1958 J. H. GUYTON TRANSISTOR OSCILLATOR STARTING CIRCUIT Original Filed Nov. 14, 1955 INVENTOR.

ATTORNEY United States Patent C) 2,854,582 TRANsis'ToR OSCILLATOR STARTING CIRCUIT Original application November 14, 1955, Serial No. 546,547. Divided and this application October 23, 1957, Serial No. 691,824.

2 Claims. (Cl. 250-36) This invention relates to transistor oscillator circuits and'more particularly to means for initiating oscillations in such circuits. This is a divisional patent application of my copending application Serial Number 546,547, filed November 14, 1955.

Since the advent of transistor or semiconductor current amplifying devices in the field of electronics many circuit arrangements have been developed for utilizing such devices as oscillation generators. Oscillations are generally initiated by a starting current in the input circuit and in the feedback type oscillator the oscillations are sustained by energy supplied from the output circuit. It has been a common practice to supply the starting current from a voltage source separate from the output voltage source or from a voltage divider impedance connected to the output voltage source. These prior art arrangements are subject to disadvantages in that they require numerous circuit elements or impose limitations upon the circuit arrangements.

Accordingly, it is an object of this invention to provide an improved starting circuit for a transistor oscillator requiring a minimum of circuit elements.

A more particular object is to provide a transistor oscillator starting circuit which is energized from the output circuit voltage source.

A further object is to provide a transistor oscillator having a singlevoltage source adapted to initiate and sustain oscillations in the output circuit.

An additional object is the provision of a transistor oscillator having a single voltage source adapted to initiate oscillations upon energization of the output circuit.

In the accomplishment of these objects there is provided circuit means for accumulating a starting voltage from the output circuit voltage source during non-operating intervals and for supplying the starting voltage to the input circuit to initiate oscillations for the operating intervals.

A more complete understanding of the invention may be had from the detailed description that follows taken with the accompanying drawings in which:

Figure 1 is a schematic diagram showing one embodiment of the invention in a transistor oscillator.

'Figure 2 is a schematic diagram of a transistor oscillator in a push-pull connection embodying the invention.

Referring now to the drawings, there is shown an illustrative embodiment of the invention in a feedback type transistor oscillator suitably employed as an inverter for developing a relatively high direct voltage from a low direct voltage source. The oscillator of Figure 1 com prises a transistor having emitter electrode 14, collector electrode 24 and base electrode 12. The transistor employed in the illustrative embodiment is of the P-N-P configuration in which the emitter and collector are of P type material having holes as the majority carriers and the base is of N type material having electrons as majority carriers. It will be apparent that the transistor may be either the point contact or junction type and that an N-P-N configuration may be employed, if desired, with a suitable reversal of polarities.

The input circuit of the transistor 10, extending between the base electrode 12 and emitter electrode 14, includes a base circuit resistor 16 having one terminal electrically connected to the base electrode 12 and the other terminal connected to one terminal of a feedback winding 18 of transformer 20. The input circuit is completed by a connection of common terminal or tap 22 on winding 18 to emitter electrode 14. The output circuit of the transistor 10, extending between collector electrode 24 and emitter electrode 14 includes'a direct voltage source such as the battery 26 having the negative terminal connected to the collector electrode 24 and to a point of reference potential or ground 28. The positive terminal of battery 26 is connected through a starting switch 30 to one terminal of a transformer primary winding 32. The output circuit is completed by connection of winding 32 from common terminal 22 to the emitter electrode 14. The transformer windings provide a feedback path, through the inductive coupling between the winding 32 in the output circuit and thevwinding 18 in the input circuit, to sustain oscillations in the output circuit.

The oscillating voltage of the output circuit may be suitably transformed to any desired voltage level by the secondary winding 34 of transformer 20. A rectifier comprising a duo-diode 36 has one plate electrode 38 connected to one terminal of the secondary winding 34 and the other plate electrode 40 connected to the other terminal of the secondary winding. A center tap 42 of the transformer secondary winding 34 is connected through switch 30 and battery 26 to ground 28. The rectifier circuit is completed by the connection of cathode electrode 44 through filter condenser 46 to ground 48. Any suitable load or utilization device represented by resistor 50 is connected between cathode 44 and ground 52.

The oscillation circuit thus far described is disclosed and claimed in copending application Serial No. 512,176, filed May 31, 1955, in the names of Guyton et al. and assigned to the same assignee as the present invention.

The starting circuit, in accordance with this invention, for initiating oscillations comprises voltage accumulating means or condenser 54 having one terminal connected by conductor 56 to the positive terminal of battery 26 and the other terminal connected by conductor 58 to the base electrode 12 of transistor 10. A series circuit is thus established including the battery 26, condenser 54, transistor base electrode 12 and collector electrode 24.

In operation of the circuit of Figure 1, oscillations are initiated by a momentary or transient starting current between emitter electrode 14 and base electrode 12 in the input circuit. Consider first the oscillator in the nonoperating condition, which obtains with starting switch 30 open because no voltage is applied between the emitter electrode 14 and collector electrode 24. In this nonoperating condition a series circuit is maintained which extends from the positive terminal of battery 26 through condenser 54, to transistor base electrode 12 and thence to transistor collector electrode 24 to the negative terminal of battery 26. The resistance between base electrode 12 and collector electrode 24 is typically of a magnitude in the range of one megohm for junction type transistors and of a few kilohms for point contact type transistors. Thus during a non-operating interval of the oscillator, a leakage or charging current will flow in the series circuit just described until the condenser 54 assumes the voltage of the polarity indicated which is approximately equal to the voltage of battery 26.

7 Upon energization of the output circuit by closing starting switch 30 a discharge path, which includes a portion of the input circuit, is established for condenser 54. A current is caused to flow from one terminal of the charged condenser 54 through conductor 56, switch 30 and transformer winding 32 to emitter electrode 14 and thence to base .electrode 12 and the other terminal of condenser 54. It is noted that a parallel path for the discharge current exists from the terminal 22 through winding 18 and resistor 16, and conductor 58 to the condenser 54. The impedance presented by the winding 18 and by the resistor 16 limits the current flow through this parallel path. As a result, a substantial part of the discharge or starting current flows from the emitter elecrode 14 to the base electrode 12.

The starting current is, of course, in the low resistance direction from emitter electrode 14 to the base electrode 12. This effectively decreases the impedance between the emitter electrode 14 and collector electrode 24 and an initially small current flows in the output circuit. An induced voltage with the polarity indicated appears across winding 32 and by virtue of inductive coupling a feedback voltage with like polarity is developed across winding 13. This feedback voltage causes additional current to flow from the emitter to the base electrode and as a result, the current from the emitter to the collector electrode increases. This action is cumulative and the current in the output circuit increases exponentially to a limiting value determined by the circuit parameters. At this limiting value the feedback voltage decreases toward zero and will no longer sustain emitter to collector current and the output circuit current commences to decrease abruptly terminating the first half-cycle. At this point, the magnetic field of transformer collapses reversing the voltage polarity across the transformer windings during the second half-cycle. Following the second half-cycle the condenser 54 delivers a starting current in the low impedance direction from the emitter electrode 14 to the base electrode 12 to initiate the second cycle of oscillation. This action is repetitive in the manner described to sustain continued oscillations.

The alternating voltage developed across winding 32 appears at a transformed value across transformer secondary winding 34. On alternate half-cycles the plate electrodes 38 and 48 are rendered conductive by the voltage developed across secondary winding 34. Thus, a full wave rectified voltage appears across filter condenser 46 and load device 56. it is noted that the battery 26 is connected between the center tap 42 of transformer secondary winding 34 and ground 28. This connection combines the battery voltage and transformer secondary voltage additively and increases the net output voltage accordingly. The over-all efiiciency of the system is thus improved by reason of the increased voltage.

In Figure 2 the inventive starting circuit is embodied in a push-pull connected transistor oscillator circuit. In this illustrative embodiment the oscillator comprises a pair of transistors 66 and 6b of 'PNP configuration, a voltage source such as battery 62 and an output transformer 64 having a primary winding 66 and secondary winding 68. The transistor 66 includes an emitter electrode 70, collector electrode 72, and base electrode 74. The input circuit of transistor 66 extending from emitter electrode 70 to base electrode 74 includes that portion of winding 66 between intermediate tap 7 6, which is conductively connected to the emitter electrode, and terminal 78, which is connected to base electrode 74. The output circuit extends from collector electrode 72 to the negative terminal of battery 62 and from the positive terminal through starting switch 80 to the center tap 32 on winding 66. The output circuit is completed from the center tap 82 through winding 66 to intermediate tap 76 and thence to emitter electrode 76. Transistor 66 includes an emitter electrode 76, collector electrode '72, and base electrode '74. The input circuit between emitter and base electrodes includes that portion of winding 66 between intermediate tap 84, which is connected to emitter electrode 70', and terminal 86, which is connected to base electrode 74. The output circuit extends from collector electrode 72' to the negative terminal of battery 62 and from positive terminal through switch 80 to center tap 82 on winding 66. The output circuit is completed through a portion of winding 66 by connection of intermediate tap 84 to emitter electrode 7 t).

A starting circuit for initiating oscillations comprises the connection of the condenser 88 between the positive erminal of battery 62 and the tap 84 on winding 66. With switch 80 open, a charging circuit is maintained for condenser 83 which extends from battery 62, through condenser 88 to tap 84 and thence through winding 66 to terminal 7 8, through transistor 60 from base electrode 74 to collector electrode 72, and back to battery 62. A similar charging circuit also exists from battery 62 through condenser 88 to tap 84 and through winding 66 to terminal 86 and thence through transistor 60 between base electrode 74' and collector electrode 72, back to battery 62. Thus the condenser 88 maintains a charge of the polarity indicated during non-operating periods of the oscillator circuit.

Operation of the oscillator is initiated by closing the starting switch 80 which completes a discharge path for the condenser 88 extending from one terminal thereof through switch 86 to center tap 82 and through winding 66 to intermediate tap 84, and thence to the other terminal of condenser The transient current delivered through this path develops an induced voltage in winding 66 which appears between tap 76 and terminal 78 and is applied across emitter electrode 70 and base electrode 74 in the low impedance direction. This causes a starting current to flow in the input circuit of transistor 60 which permits an output current to flow from battery 62 through switch S0, winding 66 between taps S2 and 76, and thence from emitter electrode 70 to collector electrode 72 and back to battery 62. This current in the output circuit of transistor 60 develops an induced voltage in winding 66 which is applied to the input circuit of transistor 60'. This voltage appearing between tap 84 and terminal 86 is applied across emitter electrode 70' and base electrode 74' in the low impedance direction and causes a starting current for transistor 6%. This starting current permits an output current to flow from'battery 62 through switch 80 to center tap 82 and through winding 66 to intermediate tap 84 and thence through emitter electrode 70 and collector electrode 72' to battery 62.

The first cycle of oscillation is thus completed by alternate conduction of the transistors 60 and 60. The conducticn. of the output circuit of the transistor 60' through winding 66 develops a starting voltage and current which is applied to the input circuit of transistor 60 to initiate the second cycle of oscillation. This action is repetitive at the oscillation frequency to maintain sustained oscillations which develop an alternating output voltage across the terminals of secondary winding 68 for any desired utilization. The oscillation is interrupted and initiated at will by opening or closing, respectively, the starting switch 80.

Although the description of this invention has been set forth with respect to particular embodiments, it is not to be construed in a limiting sense. Many modifications and variations within the spirit and scope of the invention will now occur to those skilled in the art. For a definition of the invention, reference is made to the appended claims.

I claim:

1. A push-pull connected transistor oscillator comprising a pair of transistors each having emitter, collector, and base electrodes, a transformer having a primary and a secondary winding, each of said base electrodes being connected to a different terminal of the primary winding, each of said collector electrodes being connected through a common voltage source and starting switch to a center tap on the primary winding, each of said emitter electrodes being connected to a different intermediate tap on the primary winding, a condenser connected between one of said intermediate taps and a point intermediate said voltage source and switch whereby the condenser assumes a charge from the battery during intervals when said switch is open.

2. A push-pull connected transistor oscillator comprising first and second transistors each having emitter, collector, and base electrodes, a transformer including primary and secondary windings, each of said transistors having an output circuit extending between its collector and emitter electrodes and including a difierent portion of the primary winding, a common voltage source and starting switch; each of said transistors having an input circuit extending between its emitter and base electrodes and including a portion of the primary winding common to the output circuit of the other transistor whereby sustained oscillations may be generated in said secondary winding by alternate conduction of the first and second transistors, and a starting circuit to initiate oscillations comprising a condenser connected betwen a point intermediate the voltage source and switch and a point on 10 the primary winding.

No references cited. 

